Determination of Chromaticity of Plastics: Color Measurement Principles, Methods, Standards, and App

Chromaticity determination provides an objective, instrument-based quantification of the color of plastic materials using internationally standardized CIE color spaces such as CIELAB. This technical article explains the principles of plastic color measurement, typical sample types, measurable color parameters (including color difference ΔE and yellowness index), applicable ISO, ASTM, GB, and JIS test methods, and the main industrial applications. It also answers frequently asked questions to help manufacturers control color consistency and detect discoloration or yellowing in polymers.

Technical Overview

Chromaticity determination (color measurement) is the process of quantifying the color of a plastic material in numerical terms rather than relying on subjective visual judgment. Because human color perception is influenced by lighting, observer sensitivity, fatigue, and ambient conditions, instrumental colorimetry is now the preferred approach for specification, quality control, and batch release decisions in the plastics industry.

Modern color measurement is based on the colorimetric system defined by the International Commission on Illumination (CIE). A spectrophotometer or tristimulus colorimeter measures the light reflected from (or transmitted through) a specimen under a defined standard illuminant (most commonly D65, representing average daylight) and a standard observer (the CIE 1931 2° or 1964 10° supplementary observer). The measured data are reduced to tristimulus values (X, Y, Z), chromaticity coordinates (x, y), or, most commonly in industrial practice, CIELAB coordinates (L*, a*, b*) and the derived cylindrical form (L*, C*, h°), where L* represents lightness, a* the red–green axis, b* the yellow–blue axis, C* chroma (saturation), and h° hue angle.

For plastics, chromaticity data serve three main purposes: verifying that a produced lot matches an approved color standard within agreed tolerances; monitoring process variables such as pigment dispersion, residence time, and contamination; and quantifying discoloration (typically yellowing) that develops during thermal aging, UV weathering, or sterilization. Measurements are performed by accredited third-party laboratories as well as in-house quality laboratories, using calibrated instruments traceable to certified reference materials.

Testing Samples

Plastic Pellets and Granules

Natural or colored pellets can be measured directly using a special pellet cell or glass cup presented to the measurement port of a benchtop spectrophotometer. Because surface scatter and pellet packing influence the reading, multiple fillings and repeat readings are averaged. Pellet measurement is fast and useful for incoming resin inspection, although it is less reproducible than measurements on molded specimens.

Molded Plaques and Test Panels

For the highest accuracy, resin or masterbatch is compression molded or injection molded into flat plaques under controlled temperature, pressure, and cycle time. Plaques with a smooth, glossy, opaque surface and defined thickness (commonly 2–3 mm) provide the most repeatable color data and are the preferred specimen type for color approval and certification.

Films and Sheets

Transparent and translucent films are measured in transmittance mode or in reflectance mode over defined white and black backings. Specifying the backing is essential, because film color results depend strongly on the background when the material is not fully opaque.

Finished Parts and Components

Automotive trim, appliance housings, closures, medical device components, and other finished goods can be measured directly on flat areas of the part, provided the measured area is larger than the instrument aperture and the surface is clean and free of texture effects that invalidate comparison with smooth standards.

Powders and Compounded Materials

PVC compounds, masterbatch, fillers, and pigment powders are measured in powder cells or pressed into pucks with a defined surface, following a consistent packing procedure to minimize density and gloss variation between readings.

Testing Items

Typical chromaticity parameters reported for plastic materials include:

• CIE tristimulus values (X, Y, Z) and chromaticity coordinates (x, y) under specified illuminant and observer conditions.
• CIELAB color coordinates: lightness L*, red–green axis a*, yellow–blue axis b*, chroma C*, and hue angle h°.
• Total color difference ΔE*ab and its components ΔL*, Δa*, Δb*, calculated between specimen and reference standard.
• Improved color difference metrics such as ΔE00 (CIEDE2000) for tighter correlation with visual perception.
• Yellowness index (YI) and whiteness index (WI), widely used to quantify discoloration of white and near-white plastics.
• Metamerism index, evaluated by measuring under two or more illuminants (for example D65 and A) to detect color matches that fail under changed lighting.
• Transmittance color values (TINT, transmittance yellowness) for transparent specimens such as packaging films and sheets.

Testing Methods

Spectrophotometry (Reference Method)

A sphere or 45°/0° geometry spectrophotometer records the spectral reflectance (or transmittance) factor of the specimen across the visible wavelength range, typically 360–780 nm at 10 nm intervals. The spectral data are weighted with the selected illuminant and observer functions to compute tristimulus and CIELAB values. Spectrophotometry is the reference approach because the full spectral curve is retained, enabling metamerism analysis and calculation of any colorimetric index. Instrument operation and data calculation follow the CIE colorimetric system as implemented in ISO 11664 and ASTM E308, with measurement practices per ASTM E1164 and GB/T 3979.

Tristimulus Colorimetry

Tristimulus colorimeters use filtered photodetectors whose spectral responses approximate the CIE color-matching functions. They are compact, fast, and suitable for routine production checks of known materials, but they cannot resolve metamerism and are best calibrated against a working standard measured spectrophotometrically. Requirements for colorimeter measurements are covered by ASTM E1347.

Measurement Conditions and Procedure

A typical determination proceeds as follows: the instrument is zeroed and calibrated with certified black and white tiles; specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291; each specimen is measured at several positions with orientation rotation; and averaged results are reported together with the illuminant, observer, geometry, and specular mode (SCI or SCE). SCI (specular component included) is commonly used for color matching of glossy plastics, while SCE (specular component excluded) better correlates with visual assessment of surface appearance. Color tolerances and color differences are computed according to ASTM D2244 or the equivalent CIE equations.

Yellowness and Whiteness Evaluation

Yellowness index and whiteness index are calculated from the measured tristimulus values following ASTM E313, which superseded the older ASTM D1925 practice for plastics. Color change after thermal aging (ISO 11346), artificial weathering with xenon arc or UV fluorescent lamps (ISO 4892 series), or laboratory light exposure is determined as the difference in color parameters before and after exposure in accordance with ISO 4582 and its national equivalent GB/T 15596.

Visual Color Comparison

Visual comparison against approved physical standards under a standardized light booth (D65, A, and TL84 sources) remains a complementary method for appearance decisions and for validating instrumental tolerances. Standard illuminants and viewing geometries are specified in GB/T 3978 and the corresponding CIE publications.

Key Referenced Standards

• ISO 11664 / CIE S 014 series — Colorimetry (tristimulus values, CIELAB, illuminants, observers).
• ASTM E308 — Computing the colors of objects by using the CIE system.
• ASTM E1164 — Obtaining spectrometric data for object-color evaluation.
• ASTM E1347 — Color and color-difference measurement by tristimulus colorimetry.
• ASTM E313 — Calculating yellowness and whiteness indices from instrumentally measured color coordinates.
• ASTM D2244 — Calculation of color tolerances and color differences from instrumentally measured color coordinates.
• ISO 4582 / GB/T 15596 — Determination of changes in color and properties of plastics after exposure to daylight, weathering, or laboratory light sources.
• GB/T 3979 and GB/T 3978 — Measurement of object color and standard illuminants, observers, and measuring geometries.
• JIS Z 8722 — Methods of color measurement for reflecting and transmitting objects.

Applications

Chromaticity determination is applied throughout the plastics value chain:

• Incoming inspection of resins, masterbatch, and colorants against supplier color specifications.
• Color matching and formulation development in compounding, reducing off-spec material and rework.
• Statistical process control of extrusion, injection molding, and blow molding lines to keep batch-to-batch ΔE within agreed tolerances.
• Verification of color consistency across multi-component assemblies, for example automotive interior parts, appliance panels, and consumer electronics housings.
• Evaluation of yellowing resistance of white and light-colored plastics after heat aging, UV weathering, ETO or gamma sterilization, and long-term storage, expressed as ΔYI or Δb*.
• Medical and pharmaceutical packaging qualification, where product-appearance stability is part of the technical file.
• Polymer research and stabilizer development, quantifying degradation kinetics through color change.
• Certification and dispute resolution, where measurements from accredited third-party laboratories provide traceable, defensible color data.

FAQ

What color difference (ΔE) is acceptable for plastic parts?

Perception thresholds depend on color, texture, and application, but as a guideline ΔE*ab below 1.0 is usually not noticeable, 1.0–2.0 is barely noticeable, and values above 2.0–3.0 are typically visible. Many automotive and electronics specifications require ΔE*ab ≤ 1.0, and critical appearance parts may specify tolerances as tight as 0.5, often with CIEDE2000 (ΔE00) for better agreement with visual judgment.

Which illuminant and observer should be selected?

D65 illumination with the 10° standard observer is the most common condition for plastics and is the default in most specifications. Illuminant A is added when products will be viewed under warm incandescent light, and comparing results under two illuminants is the standard way to detect metamerism. The chosen condition must always be stated in the test report because coordinates are not interchangeable.

What is the difference between SCI and SCE measurement modes?

SCI (specular component included) measures color independent of surface gloss and is preferred for color formulation and batch consistency. SCE (specular component excluded) excludes mirror reflection and correlates better with how a textured or matte part looks to the eye. Glossy molded plaques are often measured in both modes to separate bulk color from surface effects.

Can transparent and translucent plastics be measured?

Yes. Transparent films and sheets are measured in total transmittance mode, reporting transmitted color and transmittance yellowness. Translucent specimens are measured in reflectance mode over defined white and black backings, which must be documented in the report, since the result depends on the backing.

Is it better to measure pellets or molded plaques?

Pellet measurement is quick and suitable for screening and incoming checks, but results are affected by pellet shape and packing. Molded plaques produced under controlled conditions give significantly better reproducibility and are strongly recommended for color approval, certification, and any measurement intended to resolve a specification dispute.

How is yellowing of plastics quantified?

Yellowing is expressed as the yellowness index calculated per ASTM E313 from tristimulus values, or as the increase Δb* or ΔYI before and after aging or weathering exposure. Test protocols combine the color measurement with an exposure standard such as ISO 4892 for xenon arc or UV fluorescent weathering, and the change is evaluated following ISO 4582.

What specimen size and preparation are required?

The specimen must be flat, uniform, and at least large enough to fully cover the instrument measurement aperture (commonly 8–30 mm diameter), with a thickness sufficient to be opaque for reflectance measurements. Specimens should be conditioned at standard laboratory atmosphere before testing, and free of dust, fingerprints, scratches, and stresses that alter color or gloss.